In the United States of America over 240 million cars and light trucks travel the roads and highways. If one includes Canada, the number of cars and light trucks increases to 259 million. While manufacturing virtually all of these vehicles, automobile manufacturers continually strive to improve motor performance and increase overall fuel efficiency. One fundamental objective to increase fuel efficiency is to replace current materials with lighter weight yet structurally similar materials.
A large percentage of vehicles, especially those manufactured within the last 10 to 15 years, utilize plastic for headlight and tail light covers. Although less common, millions of vehicles also have plastic windows and windshields. Of the various materials available to make these plastics, a majority of automotive manufacturers make these plastic parts from a polycarbonate material. Polycarbonate is an amorphous, thermoplastic molding compound discovered in the 1950's. Polycarbonate plastic qualities include high light transmission, high heat resistance, high impact strength, good dimensional stability, and low weight. Furthermore, manufacturers can form polycarbonate in a number of different shapes for parts such as headlights and tail lights.
Even though polycarbonate has many desirable qualities, this thermoplastic also has several undesirable qualities. Polycarbonate has relatively low resistance to ultraviolet light and relatively high porosity. In other words, extended exposure of polycarbonate to ultraviolet light and dirt turns the plastic from a completely clear and colorless material into one with a hazy yellow color. Over time, mere exposure to air oxidizes untreated polycarbonate plastic. When used as a material for headlights, heat from headlight lamps accelerates this oxidation process. Untreated polycarbonate surfaces also tend to absorb particulate matter from the surrounding environment. Dirt and grime particulates impregnate the porous surface of untreated polycarbonate. In the case of automotive headlights, the resulting oxidation, ultraviolet degradation, and dirt buildup on the plastic reduce the transmission of light from the headlight lamps. Essentially, the oxidized plastic and dirt tend to refract the light rays passing through the plastic medium. As more light rays are refracted away from the initial collated light pattern, the number of rays directed onto objects in the path of the vehicle is reduced, resulting in less safe driving conditions. Aside from this safety aspect, dingy and yellow headlight lamps reduce the attractiveness of the vehicle and reduce the overall vehicle resale value.
Manufacturers currently prevent degradation of the polycarbonate material by sealing the plastic with a second material, known as a hard coat. They apply a thin layer of silicone on the surface of the plastic to create this hard coat at the time the plastic is manufactured. Applying this sealant, or silicone hard coat, helps protect the polycarbonate material from ultraviolet damage, prevent oxidation of the polycarbonate surface, and prevent dirt and grime from lodging in the porous surface. Unfortunately, the silicone hard coat is not a permanent solution. While the silicone hard coat does resist oxidation, dirt, and damage from ultraviolet exposure, exposure to ultraviolet light significantly accelerates degradation of the hard coat layer, leaving vehicular polycarbonates substantially unprotected in as little as a year.
More specifically, several factors combine to separate the silicone hard coat from the polycarbonate surface. In the case of polycarbonate headlights, exposure to the environmental elements of rain, dirt, snow, and road salt, combine with heat from the headlight lamps to delaminate the silicone hard coat from the polycarbonate. Additionally, the silicone hard coat material is particularly susceptible to dents and dings from flying road debris deflected after hitting the coated lens assembly. Such damage to the hard coat layer allows air and water to penetrate the area where polycarbonate and silicone contact, accelerating the hard coat delamination process.
Once the silicone hard coat begins to separate from the polycarbonate, the unprotected polycarbonate is susceptible to degradation from ultraviolet light, dirt, water, and oxidation. As one can see, the shortcomings of silicone as a long-term sealant for polycarbonate covers are readily apparent.
Several factors impact how rapidly vehicular plastics degrade. One factor is where the vehicle is stored, either in a garage or in the sunlight. Obviously, the plastics of vehicles stored in direct sunlight degrade faster than vehicles stored in a garage. A second factor is the environment of the plastics while driving. For average vehicles, driven an average of 12,000 to 15,000 miles per year, plastics used for the headlight lenses degrade after a period of only three to five years. Vehicles exposed to roadways where high concentrations of salt are present, such as snowy roads in Colorado and Canada, or even the coastal regions along the oceans and the gulf, experience rapid plastic degradation. A third factor is the number of hours that the vehicle is on the road. Unlike noncommercial light trucks and cars, commercial trucks that log hundreds of miles each day experience rapid deterioration of the silicone hard coat and degradation of the polycarbonate headlights, with most lasting less than a year.
As mentioned, the degraded polycarbonate is both unattractive and presents the problem of unsafe driving conditions, especially when polycarbonate is used for headlights. In an attempt to remedy these problems, people have tried a variety of different solutions, all having drawbacks and limitations. One solution, which is probably the most drastic, is to simply replace the polycarbonate cover or entire headlight assembly. Certainly, this will offer an immediate solution. However new covers and assemblies for both commercial and noncommercial vehicles are quite costly. Then, the new cover or assembly will degrade just as quickly as those replaced.
Another common solution that people use to rejuvenate degraded polycarbonate is to simply remove the damaged silicone hard coat and damaged polycarbonate material. While this solution immediately remedies the appearance of the plastic, a major drawback to this solution is the fact that the exposed polycarbonate material rapidly deteriorates if not sealed with some secondary material. If one reseals the polycarbonate material with silicone, the solution offers temporary relief but the silicone degrades similar to new lenses. Alternatively, some people adopt a proactive philosophy and try to prevent degradation of the silicone hard coat and polycarbonate by applying a second sealant, such as wax. Applying a second sealant has the benefit of retarding the degradation process, but these sealants generally do not prevent the degradation. The drawbacks to this solution are the labor involved in periodically applying the second sealant and the fact that the lenses will still degrade, if only at a slower rate.
Some people attempt a two-step solution. They first strip the damaged silicone hard coat and damaged polycarbonate materials from the plastic surface. Then they apply a different sealant, such as spar varnish or polyurethane. While this solution tends to be a moderately viable solution, these secondary sealants have undesirable qualities. Many of these sealants turn yellow or brown immediately or soon after application. Other sealants become brittle when exposed to the intense heat, such as that produced by the headlight lamps. Once brittle, the sealant layer tends to break up and flake away, leaving the untreated polycarbonate material exposed.
In reviewing the state of the current art, one can see that an alternative to the silicone hard coat sealant is needed. The solution needs to protect the underlying plastic from degrading and should have a long life expectancy, without delaminating or discoloring.